Hey guys, as far as i know, this works on iOS 4.0 - iOS 4.0.2. So, here are the instructions!

1: SSH into your iPhone and goto /private/etc and backup the "fstab" file.
2: Open up fstab in TextEdit (Mac) or Notepad.
3: Add the following lines:
su
alpine (Unless you changed it to something else.)
sysctl -w hw.cpufrequency_max=???000000 (Replace ??? with the MHz Ex: 600)
sysctl -w hw.cpufrequency=???000000 (Same as above)
sysctl -w hw.cpufrequency_min=???000000 (Replace ??? with the MHz you want when the iPhone is sleeping, etc..)